Millimeter-wave radar cover

ABSTRACT

A millimeter-wave radar cover housing a millimeter-wave radar including an antenna and an electronic circuit configured to drive the antenna includes: a first site provided in front of the millimeter-wave radar to protect the millimeter-wave radar and transmit millimeter waves emitted from the antenna; and a second site including a housing space in which the antenna and the electronic circuit except for the first site are housed. The first site is made of a stacked structural body obtained by stacking at least one layer of a first constituent material having a negative relative permittivity in the frequency band of the millimeter waves and a second constituent material having a positive relative permittivity in the frequency band of the millimeter waves, and the stacked structural body is curved in a convex shape in a direction centered at an emission source of the millimeter electromagnetic waves and departing from the emission source.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of International Application No. PCT/JP2019/008595, filed on Mar. 5, 2019, which claims priority to Japanese Patent Application No. 2018-041076, filed on Mar. 7, 2018. The contents of these applications are incorporated herein by reference in their entirety.

BACKGROUND Technical Field

The present disclosure relates to a millimeter-wave radar cover, and relates to a millimeter-wave radar cover housing a millimeter-wave radar for automotive application that enables automated driving of, for example, an automobile.

Background Art

A conventional millimeter-wave radar for automotive application includes an antenna configured to receive and emit electromagnetic waves, a drive circuit configured to drive the antenna, and an electronic circuit including a power source.

The size (dimensions) of the antenna configured to receive and emit electromagnetic waves depends on the type of the antenna, but in many cases, among antennas of the same type, an antenna at a higher frequency has a smaller size because electromagnetic waves at a higher frequency have a shorter wavelength. As for the electronic circuit, progress has been made in integration and refinement along with the progress of the semiconductor technology, and downsizing has exponentially proceeded, which is not limited to this field.

However, in a conventional low-frequency radar product or an electronic circuit mounted product in an era when the semiconductor technology was yet to develop, an antenna and an electronic circuit have large sizes (dimensions), and thus are housed separately or independently disposed in the same housing in many cases.

Along with downsizing of an antenna and an electronic circuit as electric components of a millimeter-wave radar, a radar cover capable of housing these antenna and electronic circuit in the same housing has been disclosed (refer to Japanese Patent Application Laid-Open No. 2013-102512, for example).

As described above, since the antenna and the electronic circuit can be housed in the same housing or mounted close to each other, it is difficult to physically divide the antenna and the electronic circuit, and the boundary between the antenna and the electronic circuit is becoming unclear.

In designing of a housing in which these components of a millimeter-wave radar are housed, the electromagnetic wave propagation characteristic of the housing needs to be considered to effectively utilize electromagnetic waves (millimeter waves) and reduce unnecessary radiation of electromagnetic waves, which is requested for an electronic device.

When the antenna and the electronic circuit are separately housed or disposed independently from each other as in conventional cases, the housing can be designed only for each of the antenna and the electronic circuit with taken into consideration two viewpoints at effective utilization of the millimeter-wave radar and reduction of unnecessary radiation of electromagnetic waves.

However, in a recent millimeter-wave radar on which an antenna and an electronic circuit are mounted extremely close to each other due to the progress downsizing of the antenna and the electronic circuit, it is difficult to design a housing that simultaneously satisfies effective utilization of electromagnetic waves (millimeter waves) and reduction of unnecessary radiation of electromagnetic waves, which is requested for an electronic device.

Specifically, a different electromagnetic wave transmission property or screening property for the housing material is requested for each site of the millimeter-wave radar, and thus designing specifications of the housing material are different for each site. FIG. 17 lists requested designing specifications in a divided manner for sites A and B and frequency bands I and II.

Sites are divided into two in terms of transmission and screening of millimeter waves used by the millimeter-wave radar: a site A (part corresponding to a radome of the radar) at which electromagnetic waves (millimeter waves) are received and emitted; and the remaining site B in which the millimeter-wave radar is housed except for the site A.

Frequencies for which designing is considered are functionally are divided into two: a frequency band I (76.5 GHz) of millimeter wave band used as electromagnetic waves; and a wide frequency band for which reduction of unnecessary radiation of electromagnetic waves and entry of electromagnetic waves from the outside are considered, in particular, a frequency band II (equal to or lower than 1 GHz approximately) of an electromagnetic compatibility (EMC) region (noise control region) of 1 GHz or lower. The frequency band I of millimeter wave band is not limited to 76.5 GHz but may be optionally set in the range of 76 GHz to 81 GHz.

The transmissivity T of electromagnetic waves for the housing material when the sites A and B, the frequency band I (76.5 GHz), and the frequency band II (equal to or lower than 1 GHz approximately) are considered needs to be set for each of the sites A and B, the frequency band I (76.5 GHz), and the frequency band II (equal to or lower than 1 GHz approximately).

For example, in a region AI of the site A and the frequency band I (equal to or lower than 76.5 GHz approximately), millimeter electromagnetic waves need to be transmitted to achieve the radar function of the millimeter-wave radar, and thus the transmissivity T of the housing material for the electromagnetic waves is desirably “1”. The state in which the transmissivity T is “1” is the state of perfect transmission in which the housing material transmits electromagnetic waves.

In particular, in radar usage, electromagnetic waves attenuate proportionally to the square of the distance between an antenna of the millimeter-wave radar and an object in theory. Thus, in terms of the round-trip distance between emission from the antenna and returning after reflection at the object, the electromagnetic waves attenuate proportionally to the fourth power of the distance, and the transmissivity T of the housing material largely affects the performance (detection sensitivity, certainty, and accuracy) of a product.

For example, in a region BI of the site B and the frequency band I (76.5 GHz), transmission of millimeter electromagnetic waves does not need to be functionally allowed, but entry of millimeter waves needs to be prevented to avoid interference and cross talk due to electromagnetic waves from another external instrument. In other words, the transmissivity T of the housing material is desirably “0” to screen millimeter electromagnetic waves. The state in which the transmissivity T is “0” is the state of perfect screening in which the housing material transmits no electromagnetic waves.

In a region All of the site A and the frequency band II (equal to or lower than 1 GHz approximately) and a region BII of the site B and the frequency band II (equal to or lower than 1 GHz approximately), the transmissivity T of the housing material is desirably “0” to screen, without transmission, electromagnetic waves in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region for reduction of unnecessary radiation of electromagnetic waves.

At the site A of such a millimeter-wave radar housing having the above-described configuration, the transmissivity T of the housing material is desirably “1” in the frequency band I (76.5 GHz) and “0” in the frequency band II (equal to or lower than 1 GHz approximately), and completely opposite characteristics are requested for the frequency band I (76.5 GHz) and the frequency band II (equal to or lower than 1 GHz approximately).

However, no housing material simultaneously satisfies such opposite characteristics. Thus, conventionally, with taken into consideration a wavelength reduction effect due to the relative permittivity of the housing material, designing has been made in priority of the radar function by using a housing having a thickness of integral multiples of the half wavelength of electromagnetic waves to be used, to set “1” to the transmissivity T of the housing material for electromagnetic waves in the frequency band I (76.5 GHz), but with less consideration on setting “0” to the transmissivity T of the housing material for electromagnetic waves in the frequency band II (equal to or lower than 1 GHz approximately).

Thus, with a housing of the conventional material, it is possible to effectively utilize electromagnetic waves of a millimeter-wave radar, but it has been difficult to achieve sufficient reduction of unnecessary radiation of electromagnetic waves, which is requested for an electronic device.

The present disclosure is intended to solve the above-described problem and provide a millimeter-wave radar cover capable of effectively utilizing electromagnetic waves of a millimeter-wave radar, sufficiently reducing unnecessary radiation of electromagnetic waves, and preventing entering of unnecessary electromagnetic waves from the outside.

SUMMARY

To achieve the above-described intention, the present disclosure provides a millimeter-wave radar cover housing a millimeter-wave radar including an antenna and an electronic circuit configured to drive the antenna, the millimeter-wave radar cover being characterized by including: a first site provided in front of the millimeter-wave radar to protect the millimeter-wave radar and transmit millimeter electromagnetic waves emitted from the antenna; and a second site including a housing space in which the antenna and the electronic circuit except for the first site are housed, wherein the first site is made of a stacked structural body obtained by stacking at least one layer of a first constituent material having a negative permittivity in the frequency band of the millimeter waves and a second constituent material having a positive permittivity in the frequency band of the millimeter waves, and the stacked structural body is curved in a convex shape in a direction centered at an emission source of the millimeter electromagnetic waves and departing from the emission source.

In the millimeter-wave radar cover according to the present disclosure, it is preferable that the stacked structural body has an effective transmissivity close to one only when the millimeter electromagnetic waves are perpendicularly incident on the stacked structural body.

In the millimeter-wave radar cover according to the present disclosure, it is preferable that the stacked structural body is integrated with a dielectric lens having a predetermined permittivity for refracting the millimeter electromagnetic waves incident from the emission source.

In the millimeter-wave radar cover according to the present disclosure, it is preferable that the first constituent material is formed by geometrically disposing a conductive material and the stacked structural body has an effective transmissivity close to one for the frequency band of the millimeter waves when the first constituent material is stacked with the second constituent material.

In the millimeter-wave radar cover according to the present disclosure, it is preferable that the stacked structural body has an effective transmissivity close to zero for the frequency band of an electromagnetic compatibility (EMC) region lower than the frequency band of the millimeter waves.

The present disclosure can achieve a millimeter-wave radar cover capable of effectively utilizing electromagnetic waves of a millimeter-wave radar, sufficiently reducing unnecessary radiation of electromagnetic waves, and preventing entering of unnecessary electromagnetic waves from the outside.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A simplified perspective view illustrating the entire configuration of a millimeter-wave radar cover according to an embodiment of the present disclosure.

FIG. 2 A simplified cross-sectional view illustrating the millimeter-wave radar cover, and an antenna and an electronic circuit housed therein.

FIG. 3A A simplified perspective view and a cross-sectional view illustrating a first constituent material and a second constituent material of a stacked structural body forming a radome of the millimeter-wave radar cover.

FIG. 3B A simplified perspective view and a cross-sectional view illustrating a first constituent material and a second constituent material of a stacked structural body forming a radome of the millimeter-wave radar cover.

FIG. 4 A simplified perspective view illustrating the configuration of the second constituent material made of a conductive material in a lattice shape.

FIG. 5 A simplified diagram for description of transmissivity and reflectance.

FIG. 6 A graph illustrating the relation between the relative permittivity of a conductive material of electronic conduction and frequency.

FIG. 7 A graph illustrating the relation between the permittivity of a conductive material in a lattice shape and frequency.

FIG. 8 A graph illustrating calculation results of the relative permittivity and the relative permeability of a stacked structural body.

FIG. 9 A graph illustrating the transmissivity of the stacked structural body.

FIG. 10 A cross-sectional view illustrating the configuration of a curved stacked structural body.

FIG. 11A A simplified diagram for description of oblique incidence transmissivity characteristics of millimeter electromagnetic waves for the curved stacked structural body.

FIG. 11B A simplified diagram for description of oblique incidence transmissivity characteristics of millimeter electromagnetic waves for the curved stacked structural body.

FIG. 11C A simplified diagram for description of oblique incidence transmissivity characteristics of millimeter electromagnetic waves for the curved stacked structural body.

FIG. 12 A graph indicating the oblique incidence transmissivity characteristics of millimeter electromagnetic waves for the curved stacked structural body.

FIG. 13 A cross-sectional view illustrating the configuration of a dielectric lens-integrated curved stacked structural body.

FIG. 14A A simplified diagram for description of oblique incidence transmissivity characteristics of millimeter electromagnetic waves for the dielectric lens-integrated curved stacked structural body.

FIG. 14B A simplified diagram for description of oblique incidence transmissivity characteristics of millimeter electromagnetic waves for the dielectric lens-integrated curved stacked structural body.

FIG. 14C A simplified diagram for description of oblique incidence transmissivity characteristics of millimeter electromagnetic waves for the dielectric lens-integrated curved stacked structural body.

FIG. 15 A simplified diagram for description of the oblique incidence transmissivity characteristics of millimeter electromagnetic waves for the dielectric lens-integrated curved stacked structural body.

FIG. 16A A cross-sectional view illustrating the configuration of a dielectric lens-integrated curved stacked structural body in another embodiment.

FIG. 16B A cross-sectional view illustrating the configuration of a dielectric lens-integrated curved stacked structural body in another embodiment.

FIG. 17 Table listing transmissivity requested for each of sites A and B of the millimeter-wave radar cover and each of frequency bands I and II.

DETAILED DESCRIPTION Embodiment

The following specifically describes an embodiment of the present disclosure with reference to the accompanying drawings. In the description, for the purpose of illustration, the direction of arrow “a” points to the front surface side of a millimeter-wave radar cover 1 in FIGS. 1 and 2, and the direction of arrow “b” points to the back surface side thereof.

Entire Configuration of Millimeter-Wave Radar Cover

As illustrated in FIGS. 1 and 2, the millimeter-wave radar cover 1 is a housing that houses: an antenna 30 as emission source configured to receive and emit electromagnetic waves of, for example, 76.5 GHz in the frequency band (30 to 300 GHz) of millimeter waves; and an electronic circuit 40 including, for example, a drive circuit configured to drive the antenna 30 and a power source, and protects these components from the outside.

The millimeter-wave radar cover 1 includes a first site A corresponding to a radome disposed in front of the antenna 30 configured to receive and emit electromagnetic waves, and a second site B corresponding to an accommodating part having a bottomed rectangular tubular shape and including an accommodating space in which the antenna 30 and the electronic circuit 40 except for the first site A are housed.

As illustrated in FIGS. 3(A) and (B), the first site A of the millimeter-wave radar cover 1 is a three-layer stacked structural body 10 obtained by stacking a first constituent material 11 and second constituent materials 12 sandwiching the first constituent material 11 therebetween on the front surface side (the direction of arrow “a”) and the back surface side (the direction of arrow “b”).

The second site B of the millimeter-wave radar cover 1 is formed of a shield material made of metal such as iron or a composite material obtained by providing, for example, resin with metal plating, and reduces unnecessary radiation of electromagnetic waves from the electronic circuit 40 and prevents interference and cross talk due to an external electronic device. Thus, the transmissivity T of the second site B is “0” in any of a frequency band I (76.5 GHz) of millimeter waves and a frequency band II (equal to or lower than 1 GHz approximately) of an EMC region.

As described above, the first site A needs to allow passing of millimeter waves in the frequency band I (76.5 GHz) of a millimeter wave band to achieve the radar function of a millimeter-wave radar, and thus the transmissivity T of the stacked structural body 10 for electromagnetic waves at the first site A is desirably “1”. The transmissivity T of the stacked structural body 10 at the second site B is desirably “0” to screen electromagnetic waves in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region (noise control region) without transmission, thereby reducing unnecessary radiation of electromagnetic waves.

Configuration of Stacked Structural Body

Such completely opposite characteristics that the transmissivity T at the first site A is “1” in the frequency band I (76.5 GHz) of millimeter waves and “0” in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region are requested as described above. No material satisfies the opposite requests, and thus, the stacked structural body 10 as an artificial material is used in the present disclosure.

As illustrated in FIG. 3A and FIG. 3B, the stacked structural body 10 includes the first constituent material 11 and the second constituent materials 12 as a housing that simultaneously satisfies the effective transmissivity T of “1” in the frequency band I (76.5 GHz) of millimeter waves and the effective transmissivity T of “0” in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region. However, the transmissivity T of “1” or “0” is a value in theory, and values thereof that can be actually achieved and evaluated are defined as follows: the transmissivity T of “1” is a transmissivity T equal to or higher than 0.95 (−0.45 dB) and close to one; and the transmissivity T of “0” is a transmissivity T equal to or lower than 0.1 (−20 dB) and close to zero.

As illustrated in FIGS. 3(A) and (B), the stacked structural body 10 in this case has a three-layer sandwich structure in which the first constituent material 11 is sandwiched between the second constituent material 12 disposed on the front surface side (the direction of arrow “a”) and the second constituent material 12 disposed on the back surface side (the direction of arrow “b”) and is integrally formed by, for example, an adhesive. However, the stacked structural body 10 is not limited thereto, but may have a double-layer structure as long as at least one second constituent material 12 and at least one first constituent material 11 are stacked, or may have a multiple-layer stacked structural body in which a plurality of second constituent materials 12 and a plurality of first constituent materials 11 are stacked in a total of four or more layers.

As illustrated in FIG. 4, the first constituent material 11 is a conductive material of electronic conduction, such as copper (metal) having a rectangular shape as a whole, and formed in a lattice shape like a screen door by using, for example, copper metal wires. The first constituent material 11 is not limited to a rectangular shape but may have any other kind of shape such as circular shape or an ellipse shape in accordance with the shape of the radar. The conductive material of the first constituent material 11 is not limited to metal such as copper, but may be carbon, conductive macromolecule, conductive polymer, or the like, or a material provided with conductivity by mixing each of these materials (metal, carbon, conductive macromolecule, conductive polymer, or the like) into resin, rubber, elastomer, or the like.

Specifically, in the first constituent material 11, for example, the size of the lattice and the number thereof are determined by a thickness t of each frame forming the lattice, a width d of the frame, and an array interval a of the frame. The array interval a is the distance between inner ends of adjacent frames forming the lattice, but not limited thereto. The array interval a may be the intercentral distance between the centers of the frames.

Each second constituent material 12 is formed of resin (such as polyimide, polytetrafluoroethylene, or polyethylene) or dielectric such as rubber having material strength and resistance necessary as a housing of the millimeter-wave radar cover 1. Similarly to the first constituent material 11, the second constituent material 12 has a rectangular shape as a whole, and has a size same as that of the first constituent material 11 or a size slightly larger than that of the first constituent material 11 to avoid protrusion of the first constituent material 11.

As illustrated in FIG. 5, the stacked structural body 10 is disposed in a free space, which is a situation in which reflection of electromagnetic waves radiated from the antenna 30 occurs at two interfaces of a surface 10 a of the stacked structural body 10 on the front surface side (the direction of arrow “a”) and an internal surface 10 b of the stacked structural body 10.

In such a situation, the transmissivity T of the stacked structural body 10 disposed in the free space holds a relation with the reflectance Γ, which is determined by the wave impedances η of different materials such as air and the stacked structural body 10 as indicated by Expression (1) below. Specifically, the transmissivity T of the stacked structural body 10 is determined by a wave impedance η1 of the free space (air) and the equivalent wave impedance η2 of the stacked structural body 10. The reflectance Γ is given by (η2−η1)/(η2+η1).

$\begin{matrix} \begin{matrix} {T = {1 + \Gamma}} \\ {= {1 + {\left( {{\eta\; 2} - {\eta\; 1}} \right)/\left( {{\eta\; 2} + {\eta\; 1}} \right)}}} \\ {= {{2 \cdot \eta}\;{2/\left( {{\eta\; 2} + {\eta\; 1}} \right)}}} \end{matrix} & (1) \end{matrix}$

T: transmissivity

Γ: reflectance

η1: wave impedance of incident side material (air)

η2: wave impedance of radiation side material (stacked structural body 10)

Expression (1) indicates that the transmissivity T=“1” can be obtained by equalizing the wave impedance η1 of the incident side material (air) and the wave impedance η2 of the radiation side material (stacked structural body 10). This oppositely means that the transmissivity T≈“0” can be obtained when the wave impedance η2 of the radiation side material (stacked structural body 10) is smaller than the wave impedance η1.

Each wave impedance η is determined by the permittivity and permeability of the material and given by Expression (2) below. η=√(μ0·μr/ε0·εr)   (2)

μ0: vacuum permeability

μr: relative permeability

ε0: vacuum permittivity

εr: relative permittivity

Therefore, the wave impedance η1 of the incident side material (air) is given by Expression (3) below, and the wave impedance η2 of the radiation side material (stacked structural body 10) is given by Expression (4) below. η1=√(μ0·μr1/ε0·εr1)   (3) η2=√(μ0·μr2/ε0·εr2)   (4)

Since the wave impedances η1 and η2 are given by Expressions (3) and (4) in this manner, the transmissivity T is determined by the relative permeability μr1 and the relative permittivity εr1 of the free space (air) and the equivalent relative permeability μr2 and the equivalent relative permittivity εr2 of the stacked structural body 10.

In Expression (3), when the relative permeability μr1 and the relative permittivity εr1 of the free space (air) are both taken to be substantially “1” and the equivalent relative permeability μr2 and the equivalent relative permittivity εr2 of the stacked structural body 10 have equal values, the wave impedance η1 of the air and the equivalent wave impedance η2 of the stacked structural body 10 have equal values, and accordingly, the transmissivity T=1 can be achieved.

When the ratio of the equivalent relative permeability μr2 and the equivalent relative permittivity εr2 of the stacked structural body 10 decreases, in other words, when the relative permittivity εr2 as the denominator has a negative value and the absolute value thereof increases, the wave impedance η2 approaches to “0”, and accordingly, the transmissivity T=0 can be achieved.

However, the relative permeability μr of a non-magnetic body has a value substantially equal to one, whereas the relative permittivity εr of polytetrafluoroethylene, which is lowest as an industrial material of a typically used non-magnetic body, is two. Thus, the non-magnetic body needs to be mixed with a magnetic material to increase the relative permeability μr so that the relative permeability μr is equivalent to the relative permittivity εr. However, the magnetic material leads to a large loss of electromagnetic waves and is not suitable for the mixture nor single use.

Thus, in the present disclosure, the equivalent relative permeability μr2 and the equivalent relative permittivity εr2 of the stacked structural body 10 can set to be equal to each other by forming the stacked structural body 10 as a stack of the first constituent material 11 made of an artificial material having a negative relative permittivity εr in the frequency band I (76.5 GHz) of millimeter waves and the second constituent materials 12 each having a normal positive relative permittivity εr in the frequency band I (76.5 GHz) of millimeter waves.

The stacked structural body 10 needs to be formed such that the equivalent relative permittivity εr of the stacked structural body 10 is equal to the equivalent relative permeability μr=1 in the frequency band I (76.5 GHz) of millimeter waves and the equivalent relative permittivity εr of the stacked structural body 10 is negative and has a large absolute value in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region (noise control region).

The first constituent material 11 forming the stacked structural body 10 is made of, for example a conductive material of electronic conduction typically made of metal. However, the first constituent material 11 does not necessarily need to be made of metal but may be made of a conductive material of electronic conduction, not ion conduction nor hole conduction. Examples of the conductive material other than metal include carbon, conductive macromolecule, conductive polymer, or the like, or a material provided with conductivity by mixing each of these materials into resin, rubber, elastomer, or the like.

The relative permittivity εr of the conductive material used for the first constituent material 11 is described based on a Drude model as an electronic conduction model, and has a positive value at the frequency f (f≥fp) equal to or higher than a plasma frequency fp or a negative value at the frequency f (f<fp) lower than the plasma frequency fp as illustrated in FIG. 6. In the Drude model, the relative permittivity εr of metal is given by the electron mass, electric charge, and the number of conduction electrons, and the plasma frequency fp is a frequency at which the relative permittivity εr is zero.

In this case, the plasma frequency fp of the metallic conductive material used for the first constituent material 11 is typically in a frequency band of a light region, and thus, as illustrated in FIG. 7, the plasma frequency fp is set to be near the regions of microwaves, millimeter waves, and terahertz waves so that the frequency band I (76.5 GHz) of millimeter waves is slightly lower than the plasma frequency fp and the relative permittivity εr has a negative value smaller than zero. In this case, in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region (noise control region), the relative permittivity εr is set to have a negative value equal to or smaller than −500.

The plasma frequency fp in the frequency band of the light region is set to be near the frequency band I (76.5 GHz) of the millimeter wave region as follows: the number of conduction electrons in a conductive material is restricted (decreased) to set the plasma frequency fp of the conductive material made of metal in the light region to be near the millimeter wave region.

Specifically, the number of conduction electrons can be restricted by decreasing the physical dimension and area of the conductive material to reduce the number of conduction electrons in the entire conductive material. Specifically, the restriction can be achieved by forming the first constituent material 11 in a lattice shape as illustrated in FIG. 4 to geometrically dispose the conductive material. In other words, the number of electrons can be physically restricted by reducing the area of the first constituent material 11.

The first constituent material 11 does not necessarily need to be a lattice made of metal (hereinafter also referred to as a “metal lattice”). For example, the first constituent material 11 can be obtained by printing a copper foil pattern on the surface of a polyimide film and then forming the printed film into a lattice shape by etching. The material of the first constituent material 11 and the manufacturing method thereof may be any material and any manufacturing method with which the entire number of conduction electrons can be restricted to obtain a desired relative permittivity εr.

FIG. 7 illustrates a calculation result of the relative permittivity εr of the first constituent material 11 when each frame forming the lattice has a thickness t of 0.2 mm, a frame width d of 0.06 mm, and an array interval a of 1.2 mm. When the number of electrons is restricted by forming the first constituent material 11 in a lattice shape to reduce the entire area, as illustrated in FIG. 8, the equivalent relative permittivity εr of the stacked structural body 10 can be made equal to the equivalent relative permeability μr=1 and the plasma frequency fp in the frequency band of the light region can be set to be near the frequency band I (76.5 GHz) of the millimeter wave region. As a result, the relative permittivity εr of the first constituent material 11 in the frequency band I (76.5 GHz) depends on the permittivity ε of each second constituent material 12 stacked thereon, but is set to have a designed value of the relative permittivity εr at smaller than zero and greater than −25, and preferably have a negative value equal to or smaller than −5, which is between −1 and −10 inclusive approximately. The relative permittivity εr in the frequency band II (equal to or lower than 1 GHz approximately) depends on the permittivity ε of the stacked second constituent material 12, but is set to have a negative value equal to or smaller than −500.

However, the thickness t, the frame width d, and the array interval a of each frame in the lattice of the first constituent material 11 can be set as appropriate in accordance with a desired relative permittivity εr, and an optional shape such as a circular shape or a triangular shape may be selected. The disposition pattern of the lattice does not need to be uniform, but the density such as variance of the lattice may be optionally set.

Each second constituent material 12 is a dielectric having a normal positive relative permittivity εr in the frequency band I (76.5 GHz) of millimeter waves. The second constituent material 12 only needs to have material strength, workability, and various kinds of durability that are necessary for a radar cover, but desirably has a small electric loss to further improve performance. Specifically, the imaginary part ε″ when the permittivity ε of the material of the second constituent material 12 in the frequency band I (76.5 GHz) is expressed in a complex permittivity is preferably small, and is, for example, preferably 0.001 or smaller, more preferably 0.005 or smaller.

In this manner, the stacked structural body 10 is formed by stacking at least one layer of the first constituent material 11 having a negative relative permittivity εr (equal to −5 or smaller) in the frequency band I (76.5 GHz) of millimeter waves, and at least one layer of the second constituent material 12 having a positive relative permittivity εr in the frequency band I (76.5 GHz) of millimeter waves.

Accordingly, as illustrated in FIG. 8, the equivalent relative permittivity εr of the stacked structural body 10 having a three-layer structure including the two second constituent materials 12 and the single first constituent material 11 can be made equal to the equivalent permeability μr of the stacked structural body 10, which is equal to one, in the frequency band I (76.5 GHz) of millimeter waves, and the equivalent relative permittivity εr of the stacked structural body 10 can be made negatively large (−500 or smaller) in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region.

Specifically, it is desirable to set the equivalent relative permittivity εr of the stacked structural body 10 in the frequency band I (76.5 GHz) to be substantially one, specifically, 0.9 to 1.3, more preferably, 1.0 and set the equivalent relative permittivity εr of the stacked structural body 10 in the frequency band II (equal to or lower than 1 GHz approximately) to be −370 or smaller, preferably, −500 or smaller.

Accordingly, the equivalent relative permeability μr and the relative permittivity εr of the stacked structural body 10 become equal to each other, and the wave impedance η1 of the air and the equivalent wave impedance η2 of the stacked structural body 10 become equal to each other. As a result, as illustrated in FIG. 9, the stacked structural body 10 can obtain the effective transmissivity T=1 in the frequency band I (76.5 GHz).

Simultaneously, as the ratio of the equivalent relative permeability μr and the relative permittivity εr of the stacked structural body 10 decreases when the relative permittivity εr2 as the denominator has a negative value with a large absolute value, the wave impedance η2 approaches to “0”, thereby achieving the effective transmissivity T=0 in the frequency band II (equal to or lower than 1 GHz approximately).

Operations and Effects

In the millimeter-wave radar cover 1 with the above-described configuration, the stacked structural body 10 obtained by stacking at least one layer of the first constituent material 11 having a negative relative permittivity in the frequency band I (76.5 GHz) of millimeter waves and at least one layer of the second constituent material 12 having a positive relative permittivity in the frequency band I (76.5 GHz) of millimeter waves is used as a radome at the first site A.

The stacked structural body 10 protects the antenna 30 and the electronic circuit 40 inside through the second constituent materials 12 and achieves the transmissivity T=1 in the frequency band I (76.5 GHz) of millimeter waves, and the transmissivity T≈0 in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region.

Accordingly, the millimeter-wave radar cover 1 can transmit millimeter electromagnetic waves from the antenna 30 without electric attenuation due to the stacked structural body 10, and receive reflected waves thereof without electric attenuation due to the stacked structural body 10. Simultaneously, the millimeter-wave radar cover 1 can, through the stacked structural body 10, reduce unnecessary radiation of electromagnetic waves in the EMC region and prevent interference and cross talk due to electromagnetic waves from another external instrument.

EXAMPLE

In a specific configuration of the stacked structural body 10, for example, the first constituent material 11 was made of a metal lattice of copper, each second constituent material 12 was made of polyimide, the first constituent material 11 and the second constituent material 12 each had a size of 200×200 mm, and one of the second constituent materials 12, the first constituent material 11, and the other second constituent material 12 were stacked in the stated order to achieve a three-layer stacked structure.

The first constituent material 11 was produced by etching a copper foil having a thickness t of 0.08 mm into a lattice shape having a line width d of 0.06 mm, an array interval a of 1.2 mm in the longitudinal direction, and an array interval a of 1.2 mm in the lateral direction. In this case, the plasma frequency fp of the relative permittivity εr of the first constituent material 11 in the frequency band of the light region can be set to be near the frequency band I (76.5 GHz) of the millimeter wave region. Each second constituent material 12 was made of polyimide having a complex relative permittivity of 3.25-j0.001 in the frequency band I (76.5 GHz) of millimeter waves, and had a thickness of 0.31 mm.

The one second constituent material 12, the first constituent material 11, and the other second constituent material 12 were sequentially stacked and subjected to press molding under a pressure of 60 tons by using an adhesive to form the stacked structural body 10 having a three-layer stacked structure. As a result, the thickness of the formed stacked structural body 10 was 0.62 mm. In this state, the first constituent material 11 was crushed between the two second constituent materials 12 with no air between the three layers.

In this case, for example, the relative permittivity εr and the relative permeability μr of each of the first constituent material 11 and the second constituent materials 12 of the stacked structural body 10 having a three-layer stacked structure including the adhesive were adjusted as appropriate to achieve the transmissivity T=1 in the frequency band I (76.5 GHz) of millimeter waves and the transmissivity T≈0 in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region.

A result of measurement of the equivalent relative permittivity εr and the equivalent relative permeability μr of the stacked structural body 10 is as illustrated in FIG. 8. In addition, as illustrated in FIG. 9, the transmissivity T of the stacked structural body 10 achieves the transmissivity T=1 in the frequency band I (76.5 GHz) of millimeter waves and the transmissivity T≈0 in the frequency band II (equal to or lower than 1 GHz approximately) of the EMC region.

Configuration of Curved Stacked Structural Body

With such a millimeter-wave radar cover 1, it is possible to emit millimeter waves through the stacked structural body 10 having a three-layer structure and detect the moving speed, distance, size, and the like of a physical body in the vicinity, in particular, on the front side (object such as a front vehicle or a human) based on the intensity of reflected waves of the millimeter waves, the time difference between the reflected waves, and the like.

In practice, when a physical body (object) on the front side is to be detected, any object on the front side is scanned with millimeter electromagnetic waves in the horizontal direction by a physical (mechanical scanning) or electrical (for example, digital beam forming or array antenna) method. The scanning method is not particularly limited, but the direction of emission may be changed by physically moving the antenna 30, or the direction of emission may be changed by the electrical method without physically moving the antenna 30.

Accordingly, millimeter electromagnetic waves emitted in a desired direction are reflected by an object in the desired direction and then can be received by the antenna 30, and thus it is possible to detect a physical body (object) existing in a certain range on the front side in the horizontal direction.

However, there can be incoming millimeter waves from directions other than the desired direction when reflected waves from the physical body (object) are received. Examples of the incoming millimeter waves include what is called multipath waves, the propagation paths of which are different from each other due to the shape of the physical body (object) and a nearby reflective object (such as a guardrail, another vehicle, or the ground), and emitted waves and reflected waves from a millimeter-wave radar other than a millimeter-wave radar of the own vehicle and a millimeter-wave radar for automotive application.

In a typical millimeter-wave radar, the beam width of the antenna 30 is narrowed to increase the resolution in the horizontal direction, and thus it is unlikely to receive millimeter electromagnetic waves from directions other than a desired direction by the antenna 30, but entering of incoming millimeter electromagnetic waves into the millimeter-wave radar increases background noise and affects the detection sensitivity.

Thus, as illustrated in FIG. 10, the millimeter-wave radar uses a curved stacked structural body 10W having a configuration for preventing millimeter electromagnetic waves from directions other than a desired direction from being incident on the millimeter-wave radar upon emission of millimeter electromagnetic waves in the desired direction when a physical body (object) in the horizontal direction is to be detected.

The curved stacked structural body 10W has a basic structure identical to that of the stacked structural body 10 described above, and is a three-layer structure product obtained by stacking the first constituent material 11 and the second constituent materials 12 sandwiching the first constituent material 11 therebetween on the front surface side (the direction of arrow “a”) and the back surface side (the direction of arrow “b”). A large difference between the curved stacked structural body 10W and the stacked structural body 10 is that the curved stacked structural body 10W is entirely curved in a convex shape in an external direction departing toward the front surface side (the direction of arrow “a”) from the antenna 30 as the emission source of the millimeter-wave radar.

The curved stacked structural body 10W is characterized by having such oblique incidence transmissivity characteristics that the transmissivity T is high when millimeter electromagnetic waves are incident on the stacked structural body 10 in a perpendicular direction (hereinafter referred to as “perpendicular incidence”) and the transmissivity T is low when millimeter electromagnetic waves are incident at an angle other than that of the perpendicular incidence.

Specifically, the curved stacked structural body 10W is centered at the antenna 30 as the emission source of millimeter electromagnetic waves and curved in a convex shape in a direction departing from the antenna 30 (to the outside of the radar) so that electromagnetic waves are perpendicular to the curved stacked structural body 10W when incident in any direction from the antenna 30.

In this case, it is ideal that the curved stacked structural body 10W is curved at the curvature of a semicircular shape having a radius r0=r1 at any part centered at the antenna 30. With such an ideal curved stacked structural body 10W, millimeter electromagnetic waves emitted from the antenna 30 can be perpendicularly incident at any part of the curved stacked structural body 10W, and also millimeter electromagnetic waves perpendicularly incident on the curved stacked structural body 10W from the outside can be received by the antenna 30.

However, since millimeter reflected waves and incoming multipath reflected waves from directions other than the direction of perpendicular incidence are incident on the curved stacked structural body 10W not perpendicularly but at some angles, the transmissivity T of the curved stacked structural body 10W is low for such waves.

As illustrated in FIG. 11A to FIG. 11C, the transmissivity T is one when millimeter electromagnetic waves incident on the curved stacked structural body 10W at an incident angle of 0°, in other words, perpendicularly, and the transmissivity T is smaller than one when the incident angle is smaller or larger than 0°. The state of the incident angle of 0° is the state in which millimeter electromagnetic waves are incident in the state of a normal (at an angle along the normal) perpendicularly intersecting the tangent line of each second constituent material 12. When the curved stacked structural body 10W has a semicircular shape of the radius r0=r1, millimeter electromagnetic waves are perpendicularly incident on the curved stacked structural body 10W even when the millimeter electromagnetic waves are emitted from the antenna 30 while being tilted at an angle α.

However, although the incident angle of 0° corresponds to a normal along which millimeter electromagnetic waves perpendicularly intersect the tangent line of each second constituent material 12, it is regarded in a precise sense that the incident angle of millimeter electromagnetic waves is perpendicular to the second constituent materials 12 even when tilted to the tangent line in the range of ±2°.

In this case, the curved stacked structural body 10W does not necessarily need to have a semicircular shape having a radius r0=r1 but may be entirely thinned in, for example, an aspherical shape (elliptical shape, in this case) having a radius r0<r1 and tilted to the tangent line in the range of ±2° at most so that the degree of convex protrusion is reduced.

The curved stacked structural body 10W having such a structure can transmit millimeter electromagnetic waves emitted in a desired direction from the antenna 30 and allow the millimeter electromagnetic waves to reach an object. However, the curved stacked structural body 10W does not transmit millimeter electromagnetic waves from directions other than the desired direction nor allow the millimeter electromagnetic waves to reach the antenna 30, and thus background noise does not increase, and the detection sensitivity is not affected.

Accordingly, the millimeter-wave radar can prevent entering of millimeter electromagnetic waves in a direction in which the object does not exist by the curved stacked structural body 10W at emission for detecting a physical body (object) in the horizontal direction while the direction of emission is scanned, for example, in a wide range of ±40° approximately with respect to the antenna 30.

As illustrated in FIGS. 11 (A) to (C), millimeter electromagnetic waves from the antenna 30 are transmitted when the millimeter electromagnetic waves are perpendicularly incident on the second constituent materials 12 of the curved stacked structural body 10W, in other words, when the millimeter electromagnetic waves are incident in a direction perpendicular to each of the second constituent material 12, the first constituent material 11, and the tangent line of the second constituent material 12, but are not transmitted in other cases.

In the drawing, “transmitted (◯)” means that the transmissivity T is equal to or higher than 0.95, and “not transmitted (x)” means that the transmissivity T is lower than 0.95. However, the value of the transmissivity T is not limited thereto but may be optionally set with taken into account usage and the detection accuracy on the measurement side.

Specifically, as illustrated in FIG. 12, the transmissivity T is substantially equal to one when the incident angle of millimeter electromagnetic waves on the curved stacked structural body 10W is 0°, but the transmissivity T is equal to 0.93 or is lower than 0.8 when the incident angle of millimeter electromagnetic waves is shifted by 5° or 10°, respectively.

The curved stacked structural body 10W made of a three-layer structural body having such oblique incidence transmissivity characteristics can be achieved by appropriately setting the thickness and permittivity of the dielectric in the second constituent materials 12 and the thickness t, the line width d, and the array interval a in the longitudinal and lateral directions of the metal lattice in the first constituent material 11.

In particular, the thickness t of the metal lattice of the first constituent material 11 is important to achieve desired oblique incidence transmissivity characteristics and may be 0.5 mm to 2.0 mm, more preferably 0.8 mm to 1.5 mm, and other dimensions may be designed values as needed.

In addition, it is possible to use a curved stacked structural body 100W of a dielectric lens-integrated type (hereinafter also referred to as “dielectric lens-integrated curved stacked structural body”) having a configuration with which millimeter electromagnetic waves from directions other than a desired direction are not incident on the millimeter-wave radar at emission of millimeter electromagnetic waves in the desired direction as illustrated in FIG. 13, similarly to the curved stacked structural body 10W.

The dielectric lens-integrated curved stacked structural body 100W is a structural body in which a dielectric lens 20 made of a convex lens having a predetermined permittivity for refracting millimeter electromagnetic waves incident from the antenna 30 is disposed between the corresponding second constituent material 12 of the curved stacked structural body 10W and the antenna 30 and integrated with the dielectric lens 20.

With the dielectric lens-integrated curved stacked structural body 100W integrated with the dielectric lens 20, millimeter electromagnetic waves are refracted inward due to the permittivity of the dielectric lens 20. Thus, the dielectric lens-integrated curved stacked structural body 100W allows perpendicular incidence of millimeter electromagnetic waves emitted from the antenna 30 at a wide angle corresponding to the refraction and can be entirely thinned like an aspherical shape (elliptical shape, in this case) having a radius r10<r11.

As illustrated in FIGS. 14(A) to (C), the transmissivity T is one when millimeter electromagnetic waves are incident on the dielectric lens-integrated curved stacked structural body 100W at the incident angle of 0°, in other words, perpendicularly, and the transmissivity T is lower than one when the incident angle is smaller or larger than 0°.

In addition, since millimeter electromagnetic waves are refracted inward due to the permittivity of the dielectric lens 20 in the dielectric lens-integrated curved stacked structural body 100W, millimeter electromagnetic waves are perpendicularly incident on the dielectric lens-integrated curved stacked structural body 100W even when the millimeter electromagnetic waves are emitted from the antenna 30 while being tilted at the angle α.

In this case, as illustrated in FIG. 15, in the dielectric lens-integrated curved stacked structural body 100W, millimeter electromagnetic waves incident on the dielectric lens 20 at a predetermined angle θ1 from the antenna 30 at an inside Rin of the millimeter-wave radar cover 1 are refracted slightly inward by the dielectric lens 20 and tilted at an angle θ2, travel through the inside of the dielectric lens 20, and then are emitted to an outside Rout through the curved stacked structural body 10W.

In addition, since the dielectric lens-integrated curved stacked structural body 100W integrally includes the dielectric lens 20, the degree (curvature) of convex protrusion can be reduced to increase thinning as compared to a case in which only the curved stacked structural body 10W is provided.

Other Embodiments

Some preferable embodiments of the present disclosure are described above, but the present disclosure is not limited to the millimeter-wave radar cover 1 according to the above-described embodiments and includes all aspects included in the concept and claims of the present disclosure. In addition, configurations may be selectively combined as appropriate to achieve at least part of the above-described problem and effect. For example, the shape, material, disposition, size, and the like of each component in the above-described embodiments may be changed as appropriate depending on a specific use aspect of the present disclosure.

Although the case in which the dielectric lens-integrated curved stacked structural body 100W integrated with the dielectric lens 20 made of a convex lens is used is described in the above-described embodiments, the present disclosure is not limited thereto but, for example, a dielectric lens-integrated curved stacked structural body 200W integrated with a dielectric lens 21 made of a concave lens as illustrated in FIG. 16A and FIG. 16B may be used.

Specifically, as illustrated in FIG. 16A, the dielectric lens-integrated curved stacked structural body 200W is an integration structural body in which the dielectric lens 21 made of a concave lens having a predetermined permittivity for refracting millimeter electromagnetic waves incident from the antenna 30 through the curved stacked structural body 10W is stacked on the second constituent material 12 on the outer periphery side in the curved stacked structural body 10W. The dielectric lens 21 as the concave lens is curved at a curvature same as that of the second constituent material 12 of the curved stacked structural body 10W.

As illustrating in FIG. 16B, in the dielectric lens-integrated curved stacked structural body 200W, millimeter electromagnetic waves are incident from the antenna 30 in a direction perpendicular (at an angle θ1=0°) to the tangent line of the second constituent material 12 of the curved stacked structural body 10W on the inner periphery side are refracted outward by an angle θ2 (θ2>θ1) as passing through the dielectric lens 21 and emitted to the outside Rout. Thus, the dielectric lens-integrated curved stacked structural body 200W allows millimeter electromagnetic waves to be emitted at a wide angle because of the integration with the dielectric lens 21 made of a concave lens, and also can be entirely further thinned.

INDUSTRIAL APPLICABILITY

A millimeter-wave radar cover of the present application disclosure is not only for automotive application to an automobile or the like, and can be used not only in the fields of transport means such as railway, aircraft, and ship, but also in the fields of electric and electronic devices and other industrial machines. 

What is claimed is:
 1. A millimeter-wave radar cover housing a millimeter-wave radar including an antenna and an electronic circuit configured to drive the antenna, the millimeter-wave radar cover comprising: a first site provided in front of the millimeter-wave radar to protect the millimeter-wave radar and transmit millimeter electromagnetic waves emitted from the antenna; and a second site including a housing space in which the antenna and the electronic circuit except for the first site are housed, wherein the first site is made of a stacked structural body obtained by stacking at least one layer of a first constituent material having a negative permittivity in the frequency band of the millimeter waves and a second constituent material having a positive permittivity in the frequency band of the millimeter waves, and the stacked structural body is curved in a convex shape in a direction centered at an emission source of the millimeter electromagnetic waves and departing from the emission source.
 2. The millimeter-wave radar cover according to claim 1, wherein the stacked structural body has an effective transmissivity close to one only when the millimeter electromagnetic waves are perpendicularly incident on the stacked structural body.
 3. The millimeter-wave radar cover according to claim 1, wherein the stacked structural body is integrated with a dielectric lens having a predetermined permittivity for refracting the millimeter electromagnetic waves incident from the emission source.
 4. The millimeter-wave radar cover according to claim 1, wherein the first constituent material is formed by geometrically disposing a conductive material and the stacked structural body has an effective transmissivity close to one for the frequency band of the millimeter waves when the first constituent material is stacked with the second constituent material.
 5. The millimeter-wave radar cover according to claim 1, wherein the stacked structural body has an effective transmissivity close to zero for the frequency band of an electromagnetic compatibility (EMC) region lower than the frequency band of the millimeter waves. 